1. Field of the Invention
The present invention relates to an electro-optic probe, which is used for observing the waveforms of a test signal based on a change in the polarization state of a light pulse caused when the light pulse generated by a timing signal is input into an electro-optic crystal which is coupled with an electric field generated by the test measuring signal.
2. Background Art
An electro-optic probe is capable of observing waveforms of a test signal based on a change in the polarization state of a laser light caused when the light pulse generated by a timing signal is input into an electro-optic crystal which is coupled with an electric field generated by the test measuring signal. When the laser light is emitted in a pulsed mode, and when the test signal is used after sampling, the measurement can be executed that has a very high time resolution. An electro-optic sampling oscilloscope is developed by the use of the electro-optic probe.
The electro-optic sampling oscilloscope (hereinafter, called EOS oscilloscope) has following advantages over the conventional sampling oscilloscope using electric probes:
(1) Measurement is easy, because the ground line is not necessary during measurement. PA0 (2) Since the metal pin disposed at the top of the electro-optic probe is insulated from the measuring circuit, a high input impedance is provided, which results in eliminating factors that disturb the conditions of the test point. PA0 (3) The use of the light pulse allows carrying out wide band measurement reaching to the GHz order.
The structure of the conventional electro-optic probe used in the measurement of signals by the EOS oscilloscope will be described with reference to FIG. 2. In the electro-optic probe shown in FIG. 2, a probe head 3 made of an insulator is mounted at the top of the probe body 2 made of metal and a metal pin 3a is inserted in the probe head 3. The reference numeral 4 denotes an electro-optic element, and a reflecting film 4a is formed on the outside surface of the electro-optic element to which the metal pin 3a contacts. The reference numeral 5 denotes a half-wave plate and the numeral 6 denotes a quarter-wave plate. The numerals 7 and 8 denote polarizing beam splitters. The numeral 9 denotes a half-wave plate, 10 a Faraday element, 12 a collimating lens, 13 a laser diode, 14 and 15 condenser lenses, and 16 and 17 denote photodiodes.
These two polarizing beam splitters 7 and 8, the half-wave plate 9, and the faraday element 10 constitute an isolator used for passing the light emitted by the laser diode 13, and for separating the light reflected by the reflecting film 4a.
Next, the optical path of the laser light emitted from the laser diode 13 will be described with reference to FIG. 2. The path of the laser light is represented by the reference symbol A.
The laser beam emitted from the laser diode 13 is converted into a parallel beam by the collimating lens 12, and input into the electro-optic element 4, after rectilinearly advancing through the polarizing beam splitter 8, the Faraday element 10, the half-wave plate 9 and the polarizing beam splitter 7, and further passing the quarter-wave plate 6 and the half-wave plate 5. The light beam input into the electro-optic element 4 is reflected by the reflecting film 4a formed at the end surface of the electro-optic element 4 facing to the metal pin 3a.
The reflected laser beam enters the photodiode 16, after passing the quarter-wave plate 6 and a half-wave plate 5, being reflected by the polarizing beam splitter 7 and is condensed by the condenser lens 14. The laser beam, which passes the polarizing beam splitter 7, enters into the photodiode 17, after being reflected by the polarizing beam splitter 8 and condensed by the condenser lens 15.
The rotation angles of the half-wave plate 5 and the quarter-wave plate 6 are adjusted such that the intensities of two laser beams entering into two photodiodes becomes identical.
Hereinafter, the operation of measurement by the use of the electro-optic probe shown in FIG. 2 is described.
When the metal pin 3a is made to contact a measuring point, a voltage is generated at the metal pin 3a and the voltage propagates to the electro-optic element, which results in causing a change of the refractive index of the electro-optic element due to the Pockels effect. After the optical property of the electro-optic element changes, and when the laser beam emitted by the laser diode 13 enters and propagates through the electro-optic element, the polarization state of the electro-optic element changes. The laser beam having the thus changed polarization state is introduced into the photodiodes 16 and 17 and is converted into an electric signal after being reflected by the reflecting film 4a and being condensed by the condenser lenses 14 and 15, respectively.
The change of the voltage applied to the measuring point is reflected as the change of the polarization state of the laser light by the electro-optic element 4, and the change of the polarization state is detected by the difference between the output from the photodiodes 16 and 17. Thus, the electric signal applied to the metal pin 3a can be measured by the difference between the outputs of the photodiodes 16 and 17.
In the electro-optic probe 1 shown above, the electric signals obtained from these photodiodes are input into an oscilloscope for processing, but it is possible to measure signals by connecting a controller for controlling the signal measurement between these photodiodes 16 and 17 and a measuring device such as a real time-type oscilloscope. Thereby, wide band measurement is facilitated by the use of the electro-optic probe 1.
As described above, it is necessary for the metal pin 3a to contact with a test point in order to execute the measurement by the use of the electro-optic probe 1. Since a change of the contact pressure of the metal pin 3a with the electro-optic probe influences on the refractive index of the electro-optic probe, it has been necessary to maintain the contact pressure at constant, which has made this measurement difficult and time-consuming.
The contact of the metal pin 3a with the test point during measurement also raises a concern that damage may be caused to the metal pin and the electro-optic probe due to the shock applied to the metal pin 3a.